Insulin binding and action have been studied in cultured human fibroblasts. Cells are derived from insulin resistant patients and from diabetics from comparison to controls. Activation of glycogen synthase, stimulation of AIB transport, and incorporation of 3H-thymidine into DNA are all being examined in these cells. Receptor defects have been found in cells from a patient with the Type A syndrome of insulin resistance and acanthosis nigricans and in an infant with leprechaunism.